Converter and a method for driving an electric machine

Abstract

A converter ( 101 ) for driving an electric machine whose stator windings are changeable to be in a low-speed configuration or in a high-speed configuration includes a converter stage ( 102 ) for supplying stator voltages to the stator windings, and a control system ( 103 ) that controls the stator windings to be in the low-speed configuration or in the high-speed configuration. The control system deactivates the converter stage during a change between the low-speed configuration and high-speed configuration and limits torque of the electric machine so that a torque limit is higher when the stator windings are in the low-speed configuration than when the stator windings are in the high-speed configuration. As the torque limit is changed when the number of series connected turns of the stator windings is changed, unwanted current transients can be reduced.

What is claimed is: 
     
         1 . A converter for driving an electric machine whose stator windings are changeable to be in a low-speed configuration or in a high-speed configuration having less series connected turns than the low-speed configuration, the converter comprising:
 a converter stage configured to supply stator voltages to the stator windings, and   a control system configured to control the stator windings to be in the low-speed configuration or in the high-speed configuration and to control the converter stage to supply the stator voltages,   
       wherein the control system is configured to limit torque of the electric machine so that a torque limit is higher when the stator windings are in the low-speed configuration than when the stator windings are in the high-speed configuration, and the control system is configured to deactivate the converter stage during a change between the low-speed configuration and the high-speed configuration and to carry out the change, and 
       wherein the control system is configured to use the following procedure during a change from a current configuration of the stator windings to a new configuration of the stator windings:
 i) set controllable switches (S 1 -S 6 ) of the converter stage into a non-conductive state, 
 ii) monitor stator currents of the electric machine, 
 iii) set all configuration switches (Cs 1 -Cs 8 ) of the stator windings into a non-conductive state after the stator currents have dropped below a threshold and wait for a first predetermined time, 
 iv) set the configuration switches (Cs 1 -Cs 8 ) of the stator windings into a position corresponding to the new configuration of the stator windings and wait for a second predetermined time, and 
 v) activate the converter stage to supply stator voltages to the stator windings. 
 
     
     
         2 . The converter according to  claim 1 , wherein the control system is configured to ramp the torque limit as a function of rotation speed, the torque limit being ramped between a higher torque limit corresponding to the low-speed configuration and a lower torque limit corresponding to the high-speed configuration. 
     
     
         3 . The converter according to  claim 1 , wherein the control system is configured to maintain a computational model of the electric machine and control rotation speed and/or torque of the electric machine based on stator currents, the stator voltages, and the computational model of the electric machine, the control system being configured to change parameters of the computational model of the electric machine during the change between the low-speed configuration and the high-speed configuration. 
     
     
         4 . The converter according to  claim 1 , wherein the control system is configured to estimate an electromotive force capable of being induced on the low speed configuration of the stator windings as a result of rotation speed of a rotor of the electric machine and a magnetic flux maintained by the rotor of the electric machine, and to allow a change from the high-speed configuration to the low-speed configuration only in a case in which the estimated electromotive force is below a threshold. 
     
     
         5 . The converter according to  claim 1 , wherein the control system is configured to prevent a change from the low-speed configuration to the high-speed configuration in a case in which less than a predetermined time has elapsed after a previous change from the high-speed configuration to the low-speed configuration, and to prevent a change from the high-speed configuration to the low-speed configuration in a case in which less than the predetermined time has elapsed after a previous change from the low-speed configuration to the high-speed configuration. 
     
     
         6 . The converter according to  claim 2 , wherein the control system is configured to maintain a computational model of the electric machine and control rotation speed and/or torque of the electric machine based on stator currents, the stator voltages, and the computational model of the electric machine, the control system being configured to change parameters of the computational model of the electric machine during the change between the low-speed configuration and the high-speed configuration. 
     
     
         7 . The converter according to  claim 2 , wherein the control system is configured to estimate an electromotive force capable of being induced on the low speed configuration of the stator windings as a result of rotation speed of a rotor of the electric machine and a magnetic flux maintained by the rotor of the electric machine, and to allow a change from the high-speed configuration to the low-speed configuration only in a case in which the estimated electromotive force is below a threshold. 
     
     
         8 . The converter according to  claim 3 , wherein the control system is configured to estimate an electromotive force capable of being induced on the low speed configuration of the stator windings as a result of rotation speed of a rotor of the electric machine and a magnetic flux maintained by the rotor of the electric machine, and to allow a change from the high-speed configuration to the low-speed configuration only in a case in which the estimated electromotive force is below a threshold. 
     
     
         9 . A method for driving an electric machine whose stator windings are changeable to be in a low-speed configuration or in a high-speed configuration having less series connected turns than the low-speed configuration, the method comprising:
 controlling the stator windings to be in the low-speed configuration or in the high-speed configuration, and   controlling a converter stage to supply stator voltages to the stator windings,   
       wherein the method comprises limiting torque of the electric machine so that a torque limit is higher when the stator windings are in the low-speed configuration than when the stator windings are in the high-speed configuration, and deactivating the converter stage during a change between the low-speed configuration and the high-speed configuration and carrying out the change, 
       wherein the method comprises using the following procedure during a change from a current configuration of the stator windings to a new configuration of the stator windings:
 i) set controllable switches of the converter stage into a non-conductive state, 
 ii) monitor stator currents of the electric machine, 
 iii) set all configuration switches of the stator windings into a non-conductive state after the stator currents have dropped below a threshold and wait for a first predetermined time, 
 iv) set the configuration switches of the stator windings into a position corresponding to the new configuration of the stator windings and wait for a second predetermined time, and 
 v) activate the converter stage to supply stator voltages to the stator windings. 
 
     
     
         10 . The method according to  claim 9 , wherein the method comprises ramping the torque limit as a function of rotation speed, the torque limit is being ramped between a higher torque limit corresponding to the low-speed configuration and a lower torque limit corresponding to the high-speed configuration. 
     
     
         11 . The method according to  claim 9 , wherein the method comprises maintaining a computational model of the electric machine, controlling rotation speed and/or torque of the electric machine based on stator currents, the stator voltages, and the computational model of the electric machine, and changing parameters of the computational model of the electric machine during the change between the low-speed configuration and the high-speed configuration. 
     
     
         12 . The method according to  claim 9 , wherein the method comprises estimating an electromotive force capable of being induced on the low speed configuration of the stator windings as a result of rotation speed of a rotor of the electric machine and a magnetic flux maintained by the rotor of the electric machine, and allowing a change from the high-speed configuration to the low-speed configuration only in a case in which the estimated electromotive force is below a threshold. 
     
     
         13 . The method according to  claim 9 , wherein the method comprises preventing a change from the low-speed configuration to the high-speed configuration in a case in which less than a predetermined time has elapsed after a previous change from the high-speed configuration to the low-speed configuration, and preventing a change from the high-speed configuration to the low-speed configuration in a case in which less than the predetermined time has elapsed after a previous change from the low-speed configuration to the high-speed configuration. 
     
     
         14 . The method according to  claim 10 , wherein the method comprises maintaining a computational model of the electric machine, controlling rotation speed and/or torque of the electric machine based on stator currents, the stator voltages, and the computational model of the electric machine, and changing parameters of the computational model of the electric machine during the change between the low-speed configuration and the high-speed configuration. 
     
     
         15 . The method according to  claim 10 , wherein the method comprises estimating an electromotive force capable of being induced on the low speed configuration of the stator windings as a result of rotation speed of a rotor of the electric machine and a magnetic flux maintained by the rotor of the electric machine, and allowing a change from the high-speed configuration to the low-speed configuration only in a case in which the estimated electromotive force is below a threshold. 
     
     
         16 . The method according to  claim 10 , wherein the method comprises preventing a change from the low-speed configuration to the high-speed configuration in a case in which less than a predetermined time has elapsed after a previous change from the high-speed configuration to the low-speed configuration, and preventing a change from the high-speed configuration to the low-speed configuration in a case in which less than the predetermined time has elapsed after a previous change from the low-speed configuration to the high-speed configuration. 
     
     
         17 . A non-transitory computer program for driving an electric machine whose stator windings are changeable to be in a low-speed configuration or in a high-speed configuration having less series connected turns than the low-speed configuration, the non-transitory computer program comprising computer executable instructions for controlling a programmable processing system to:
 control the stator windings to be in the low-speed configuration or in the high-speed configuration, and   control a converter stage to supply stator voltages to the stator windings, wherein the non-transitory computer program comprises computer executable instructions for controlling the programmable processing system to:   limit torque of the electric machine so that a torque limit is higher when the stator windings are in the low-speed configuration than when the stator windings are in the high-speed configuration, and   deactivate the converter stage during a change between the low-speed configuration and the high-speed configuration and carry out the change,   
       wherein the control system is configured to use the following procedure during a change from a current configuration of the stator windings to a new configuration of the stator windings:
 i) set controllable switches (S 1 -S 6 ) of the converter stage into a non-conductive state, 
 ii) monitor stator currents of the electric machine, 
 iii) set all configuration switches (Cs 1 -Cs 8 ) of the stator windings into a non-conductive state after the stator currents have dropped below a threshold and wait for a first predetermined time, 
 iv) set the configuration switches (Cs 1 -Cs 8 ) of the stator windings into a position corresponding to the new configuration of the stator windings and wait for a second predetermined time, and 
 vg) activate the converter stage to supply stator voltages to the stator windings. 
 
     
     
         18 . A non-volatile computer readable medium encoded with the non-transitory computer program according to  claim 17 .